The pharynx is divided into three sections: the nasopharynx, oropharynx, and laryngopharynx. Important • Fig. 20.1 Illustrations of Upper Airway Anatomy.A, Anterior view of the paranasal sinuses. B, Lateral view of the nasal passage structures demonstrating the superior, middle, and inferior turbinates and the sinus ostia. C, Lateral midsagittal section view of the head and neck, showing the three divisions of the pharynx (nasopharynx, oropharynx, and laryngopharynx) and surrounding upper airway structures.
CHAPTER 20 Introduction to the Respiratory System 435 structures within these regions include the epiglottis, vocal cords, and arytenoid cartilage which is attached to the vocal cords (see
Fig. 20.1C ). The nasopharynx (2 to 3 cm wide and 3 to 4 cm long) is the most anterior and lies behind the nose. In this region, the nose and mouth are connected via an isthmus (canals) that enables both oral and nasal breathing. Also, the nasopharynx contains small masses of lymphoid tissue (adenoids), also known as pharyngeal tonsils, which fight infections. The nasopharynx is connected to the middle ear cavity via  the eustachian tubes, which aid in equalizing pressure in  the ear to atmospheric pressure; thus they represent a drainage pathway of lymphatic fluid between the throat, nose, and ears. This network of structures provides a means of fighting infections but also is a common location for infections in the head.
The soft palate separates the nasopharynx and the oropharynx, which ends at the epiglottis. The laryngopharynx begins at the epiglottis and ends at the esophagus. Its major role is to help regulate the passage of food into the esophagus and air into the lungs. With some infections, these structures can become edematous and contribute significantly to airflow resistance. The epiglottis and arytenoid cartilage (attached to the vocal cords) cover or act as a hood over the vocal cords during swallowing. Thus under normal circumstances, the epiglottis and arytenoid cartilage function to prevent aspiration of food and liquid into the lower respiratory tract. The act of swallowing food after mastication (chewing) usually occurs within 2 seconds, and it is closely synchronized with muscle reflexes that coordinate opening and closing of the airway. Hence, air is allowed to enter the lower airways, and food and liquids are kept out. Patients with some neuromuscular diseases have altered muscle reflexes and can lose this coordinated swallowing mechanism. Such patients may become susceptible to aspiration of food and liquid, which poses a risk for pneumonia.
Lower Airways: Trachea, Bronchi, Bronchioles, and Respiratory Unit
The right lung, located in the right hemithorax, is divided into three lobes (upper, middle, and lower) by two interlobular fissures (oblique, horizontal), whereas the left lung, located in the left hemithorax, is divided into two lobes (upper, including the lingula, a tongue-like projection of the anterior aspect of the upper lobe, and lower) by an oblique fissure (